In such an exchange, it is usual to connect a given inlet line or circuit to a required outlet line or circuit via the switching network, with signalling being received and emitted under the control of exchange control units. In a time-division exchange, unlike prior art analog exchanges, tie lines cannot simply be strapped through from a given inlet to a given outlet for as long as the line is leased. Thus, in a digital exchange, semi-permanent tie line connections have to be established through the switching network, and the signalling is received in the same manner as is the signalling on lines which are switched on a demand basis. However, instead of being responded to, the signalling on a tie line must simply be forwarded through the switching network so that the telephone exchange appears to be transparent to tie lines, thereby giving the customer complete freedom in the manner in which he chooses to use the signalling channel he has leased.
This method of implementing tie lines has the drawback of using a non-negligible amount of central control resources in an exchange, in particular because providing full transparency is a difficult constraint to satisfy. Consequently, the aim of the present invention is to provide a system which enables signalling on tie lines to be forwarded without intervention by the central control means of an exchange, while still ensuring that signalling on channels which are being operated on a demand basis is still processed by the central control means.